uvi·today

Does UV Go Through Clouds, Windows, and Water?

Cloud, glass, and water all feel like shade, but each cuts the sun's heat and glare far more than its UV. Here is how much ultraviolet really gets through.

Cloud, glass, and water all feel like shade. Each one cuts the sun's heat and glare much more than it cuts the ultraviolet, so you can feel protected while most of the UV still reaches your skin. This page sets out how much gets through each one, with the source for every number.

Clouds let most of the UV through

The clearest figures come from the US Environmental Protection Agency, which publishes the cloud factors used in the US UV Index forecast. In its words, "clear skies allow virtually 100% of UV to pass through, scattered clouds transmit 89%, broken clouds transmit 73%, and overcast skies transmit 31%."

0 25 50 75 100% Share of clear-sky UV that reaches the ground Clear sky 100% Scattered cloud 89% Broken cloud 73% Overcast 31%

Source: US EPA, Learn About the UV Index. These are the cloud transmission factors used in the US UV Index forecast.

A fully overcast sky, dark enough to bring on the streetlights, still passes about a third of the clear-sky UV. A few scattered clouds take off only about a tenth. A cloudy sky also looks and feels far gentler than the UV behind it, because UVA passes through cloud much as it passes through window glass. So a grey sky removes the cue to cover up without removing most of the dose. The WHO advises sun protection whenever the UV index is 3 or above, and the live number on a city page already has cloud cover built in, which is why it can read higher than the view from the window suggests. The other factors behind a cloudy-day reading are covered in what affects the UV index.

Glass blocks the burn but not the longer-term damage

Window glass treats the two halves of the UV spectrum differently. It stops most UVB, the shorter wavelength that causes sunburn, but lets much of the UVA through. The Skin Cancer Foundation describes ordinary glass as "filtering out most UVB rays" while UVA, the longer wavelengths, pass through. This is why a long spell in a sunny window rarely reddens the skin yet still delivers a UVA dose.

Cars show the split clearly, because they use two kinds of glass. A 2024 study in Archives of Dermatological Research measured the windows of a range of vehicles and found the laminated front windshield blocked 99.25% of UVA on average, while the tempered side windows blocked about 88.78% on average, and as little as 72% in the weakest vehicle tested. UVB was stopped almost completely by every window tested. So the exposure inside a car is concentrated at the side glass, on whichever side faces the sun.

That pattern shows up in the medical record. In the United States, where the driver sits on the left, melanoma and Merkel cell carcinoma are both recorded more often on the left side of the body, and the Skin Cancer Foundation reviews the driving evidence under the heading "Driving Your Risk for Skin Cancer." Because the UVB is blocked, none of it presents as sunburn; it builds up as the slower damage tied to UVA. The same goes for a desk beside a sunny window or a long flight in a window seat over Phoenix or anywhere else the sun runs high. UV window film closes the gap: the Skin Cancer Foundation notes it can "block more than 99% of UVA and UVB light" without darkening the glass.

Water does not stop UV at the surface

Going under the surface is not the same as getting out of the sun. According to BBC Science Focus, "half a metre of water will still let 40 per cent of the UV-B through," and the same article notes that burning your back while snorkelling is easy to do. It takes several metres of clear water before UVB is largely absorbed, far deeper than most swimming happens.

Two things make water exposure easy to misjudge. The surface reflects part of the UV upward before you get in, adding to what arrives from the sky, the same ground-reflection effect that raises exposure on beaches and snow. And the water keeps you cool, removing the heat that usually tells you when you have had enough sun, so you stay in longer. Sunscreen thins after a swim as well, so a water-resistant product and reapplication matter most where the reflected glare is strongest. Places built around the water, such as Miami, Nice and Barcelona, are worth treating as high-exposure for that reason.

What this means in practice

A grey sky is a dimmer, not a shield. Overcast still passes about a third of the UV and scattered cloud almost all of it, so the index, not the brightness, tells you whether to cover up.

Behind glass you are unlikely to burn, but UVA still adds up. Side windows, office glazing and aircraft windows all pass UVA. Window film or a sleeve removes it.

Water is not a break from the sun. Forty per cent of the burning UVB reaches half a metre down, the surface adds reflected UV, and the cool water hides the dose. Use water-resistant sunscreen and reapply after swimming.

The common thread is that skin judges sun exposure by warmth and brightness, and cloud, glass and water all take those cues away while leaving much of the UV in place. The reliable guide is the index itself. Every city page on UVI.today shows the live UV index for your location with cloud cover already included, and the levels are explained in what is a safe UV index.

Sources

Live readings on UVI.today come from the Copernicus Atmosphere Monitoring Service (CAMS); the bodies behind UV measurement and sun-safety advice are listed in UV Index Official Sources.